<style>
    #fly {
        display: block;
        position: relative;
        margin: 0px auto;
        width: 50%;
        height: 40px;
        padding: 10px;
        border: none;
        border-radius: 3px;
        font-size: 12px;
        text-align: center;
        color: #fff;
        background: #ee8a65;
    }
</style>
<div id="map"></div>
<br />
<button id="fly">Fly</button>
<script>
    var start = [-74.5, 40];
    var end = [74.5, 40];
    var map = new mapboxgl.Map({
        container: 'map',
        style: 'mapbox://styles/mapbox/streets-v11',
        center: start,
        zoom: 9
    });

    var isAtStart = true;

    document.getElementById('fly').addEventListener('click', function() {
        // depending on whether we're currently at point a or b, aim for
        // point a or b
        var target = isAtStart ? end : start;

        // and now we're at the opposite point
        isAtStart = !isAtStart;

        map.flyTo({
            // These options control the ending camera position: centered at
            // the target, at zoom level 9, and north up.
            center: target,
            zoom: 9,
            bearing: 0,

            // These options control the flight curve, making it move
            // slowly and zoom out almost completely before starting
            // to pan.
            speed: 0.2, // make the flying slow
            curve: 1, // change the speed at which it zooms out

            // This can be any easing function: it takes a number between
            // 0 and 1 and returns another number between 0 and 1.
            easing: function(t) {
                return t;
            },

            // this animation is considered essential with respect to prefers-reduced-motion
            essential: true
        });
    });
</script>
